The present invention relates to the manufacture and arranging of an armature assembly for a matrix print head of the type used in serial or line printers whereby the armature is basically constructed from an outer armature ring and azimuthally distributed, inwardly directed resilient armature arms.
An armature unit or multiple armature assembly of the type to which the invention pertains is used in matrix print heads in conjunction with and as a constituing element for the drives for print elements which drives include additionally electromagnets and/or permanent magnets. In the case of a serial printer such a print element is a stylus, a needle or a wire which is propelled with a relatively large force from a normal or zero position in a direction so as to obtain printing by impacting against an ink ribbon and print material such as a sheet on a platen. As a consequence, a print dot is produced is on that sheet. The print element is fastened to a respective resiliently constructed or mounted armature arm which is, additionally, mechanically resiliently biased, for example, through a deflecting field set up by a permanent magnet. An electromagnetic field of the driver is set up for temporarily removing this permanent magnetic bias from the armature so that under the force of the previously established resilient bias, now being released, the stylus, needle or wire will be propelled forward to obtain the printing action as described. Such a magnet system is also termed a biased system or better a tension biased, magnetic system because the resilient armature arms are mechanically tensioned through the use of a permanent magnet.
Matrix print heads of the type elaborated in the previous paragraph have been developed and are, in fact, in use on a large scale and they are manufactured by mass production. Owing to the rapidly expanding market of matrix printers, the number of print heads produced is quite large, but the requirements for accuracy has remained. In the case of serial print heads an overall print speed of above 200 characters per second has been developed bearing in mind that each character is composed of a number of such print dots, and many of them will be produced sequentially rather than in parallel. On the other hand, the use-life of such a printer, in order to be marketable, should be extensive which requires a capability of writing several, even many millions of characters. It can readily be seen that mass production is not a simple production engineering task.
German printed Pat. No. 30 79 03 and European printed patent application No. 9 873 describe the manufacture of an armature unit for matrix print heads of the type mentioned above in which a rather thin armature ring is provided with radial slots to thereby establish resilient armature arms. Longitudinal components are mounted to these armature arms and fastened thereto. On the tip of these longitudinal components one arranges the print needle, wire or stylus. After assembly of the matrix print head these longitudinal components will slide in a guide slot of a mounting and cover plate. In order to permit such movement, of course a certain clearance and play is necessary between that longitudinal component and the walls bounding the guide slot. The play or clearance, however, must not be excessive. The gap width should be as small as possible in order to permit movement without "looseness". Therefore, very tight and/or very expensive tolerances have to be observed here.
Inherently a critical air gap within the magnetic circuit coil of a drive is set up which has certain parasitic side effects and may pose problems particularly if, in fact, the various slot width within a print head differ for different armatures and needles. As a consequence of such variations the dynamics of the various print wires and their propelling system is subject to (temporal, force etc.) variations. In particular then, these different slot widths may lead to different periods of time between a trigger command and impact of the needle tip upon the print medium and the subsequent full retraction. Such difference in time may, for example, result in different print actions and produces accordingly a poor appearance of the printed character.